Beginner: How do i make force field affect only a certain area of hair
Hair Dynamics is a feature that enables for the whole particle system no matter what but you can tweak which forces affect to the system.

So I think the only solution is to make two Particle Systems: one affected by the Force Field and another one that it's not. These are the steps:


Create two equal Particle Systems for your plane. They might have Hair Dynamics enabled for working with Force Fields.
Create a Vertex Group for the area you don't want to be affected by your Force Field. (Eventualy, you might need more vertices on your plane to paint a proper Vertex Group).
On the first System, go to Vertex Group and add that Vertex Group into the Density field.
On the second one, add the Vertex Group too but check the double-arrow icon on the left.
Now, you might need to tweak the number of hair on the two Systems to have the desired density.
Go back to the first System and, on the Field Weights area, set to 0 the Fields you want to avoid -depending on your Field Force. 




Now you might have a result like this, where the center area is not being affected by a Wind Force Field.Note that in this case I could just disabled Hair Dynamics for the first System.